Electronics systems incorporating digital circuitry typically need at least one reference clock. Traditional systems may use at least one crystal oscillator module, each of which typically includes a resonating crystal coupled to an active core, to provide a stable reference clock source having a reference clock frequency. However, as technology evolves and circuit geometries become smaller, an increasing proportion of circuitry is integrated into integrated circuits (ICs). As such, a crystal oscillator module may be replaced with a partially integrated crystal oscillator having a resonating crystal coupled to an active core, which has been integrated into an IC, particularly a digital IC.
Some electronics systems, such as wireless communications systems, may need at least one reference clock having a high degree of accuracy, having precise harmonic content, or both. The reference clock may be provided by a crystal oscillator. For example, when establishing communications between a mobile terminal, such as a cell phone, and a base station, such as a repeater on a cell tower, the absolute accuracy of transmitted data and received data must meet certain accuracy requirements for proper operation. To meet the accuracy requirements, an accurate reference clock may be required. In one wireless communications protocol, the absolute accuracy of a reference clock in a mobile terminal must be within five parts-per-million (PPM) under all operating conditions. Additionally, to minimize timing inaccuracy between a base station and a mobile terminal, the base station may send commands to the mobile terminal, thereby instructing the mobile terminal to make adjustments to its reference clock. Therefore, the base station and the mobile terminal may effectively form a clock recovery loop (CRL) for making adjustments to the mobile terminal's reference clock.
As in most control loops, to avoid anomalous behavior, the frequency adjustments to the reference clock must be monotonic. Monotonic behavior in control loops is well known in the art. However, in a simple description, monotonic frequency adjustments include either a frequency step that is intended to increase the frequency, which results in an actual increase in the frequency, or a frequency step that is intended to decrease the frequency, which results in an actual decrease in the frequency. A frequency step that is intended to increase the frequency which results in an actual decrease in the frequency is not monotonic. Similarly, a frequency step that is intended to decrease the frequency which results in an actual increase in the frequency is not monotonic.
Mobile terminals, such as cell phones, are typically battery-powered and may be subjected to wide supply voltage variations from the battery, wide temperature variations, or both. Additionally, resonating crystals, such as quartz crystals, may have an initial inaccuracy of a resonating frequency, may have a drift of the resonating frequency when operated over a wide temperature range, may have a drift of the resonating frequency as a crystal ages, or any combination thereof. Therefore, a crystal oscillator, particularly in a mobile terminal, may need a method for controlling the reference clock frequency having a high degree of resolution to obtain the needed frequency tolerance and a relatively wide tuning range to accommodate all of the frequency variations. In a partially integrated clock oscillator, the IC may be a digital IC; therefore, the method for controlling the reference clock frequency needs to be integrated into the digital IC as much as possible.
When the crystal oscillator is manufactured, the initial frequency inaccuracy of the crystal oscillator may be determined and then reduced or eliminated when operating the crystal oscillator by adjusting the reference clock frequency. Additionally, temperature drift characteristics of the crystal oscillator may be measured, pre-determined, or pre-estimated before using the crystal oscillator. Then, when operating the crystal oscillator, the temperature of the crystal oscillator may be measured and used with the temperature drift characteristics to adjust the reference clock frequency to reduce or eliminate the effects of temperature drift.
Traditional methods for controlling the reference clock frequency may involve a varactor diode coupled to the resonating crystal, such that a direct current (DC) bias voltage on the varactor diode is provided from a digital-to-analog converter (DAC). The DC bias voltage controls the capacitance of the varactor diode, thereby controlling the reference clock frequency by tuning an anti-resonant frequency of the crystal. Since the frequency tuning method needs a high degree of resolution and a wide tuning range, the DAC may need a wide dynamic range with high resolution. Such a DAC may be on the order of about 14-bits, which may not be suitable for integration due to large physical size, high power consumption, digital IC technology limitations, or any combination thereof.
A digital IC that includes the active core of a partially integrated crystal oscillator may be used in different designs and may need to accommodate a wide range of crystal types having variations in operating characteristics, such as an anti-resonant frequency, power loss, and quality factor. Crystals having a low quality factor may require an active core with a higher gain to ensure reliable operation and keep crystal start-up times reasonably low. However, an active core with excessive gain may have higher bias current, which may cause poor phase noise performance and unacceptable high harmonic content. Therefore, the gain of the active core needs to be within acceptable limits for proper operation. Thus, there is a need for a partially integrated crystal oscillator integrated using a digital IC incorporating a digital-control approach without a DAC, having a relatively wide tuning range with high resolution, having monotonic frequency tuning behavior, that operates over a wide temperature range, that operates over a wide supply voltage range, that is able to accommodate a wide range of crystal types having different circuit characteristics, that is compensated for initial frequency inaccuracy, that is compensated for temperature drift, or any combination thereof.